To sense the position, velocity, or orientation of a physical object, an electrical sensor frequently relies on a change in a magnetic field, which can be sensed by a variety of techniques. One technique utilizes a Hall-effect sensor, which relies on a potential difference created on opposite sides of an electrical conductor. The potential difference is created by an externally applied magnetic field perpendicular to a current flow within the sensor. Another utilizes a loop of wire, relying on Faraday's Law, to create a voltage proportional to a rate of change of a magnetic field enclosed by the area of the loop. A third technique relies on the magnetoresistive effect, which is the property of a material to change its electrical resistance in the presence of an externally applied magnetic field. Although these techniques have been applied in a range of applications, their low sensitivity to an externally applied magnetic field or issues related to low-cost manufacturing have stimulated ongoing research to identify improved field-sensing methods.
Various research efforts have focused on devices exhibiting a magnetoresistive effect. The “anisotropic magnetoresistive effect” (AMR), discovered by William Thomson in 1856, produces a small change in the electrical resistance of certain conductors in the presence of an externally applied magnetic field. Recently discovered variations of this effect produce a greater relative change in electrical resistance. One resistance-altering effect is referred to as the “giant magnetoresistive effect” (GMR), which is a quantum mechanical phenomenon observed in thin films formed of alternating ferromagnetic and nonmagnetic metal layers. Another is the “colossal magnetoresistive effect” (CMR), which is a magnetic property of some materials such as manganese-based perovskite oxides. A third is the “tunnel magnetoresistive effect” (TMR), which occurs when two ferromagnets are separated by a very thin (˜1 nm) insulator. Collectively, these magnetoresistive effects can be referred to as xMR.
Magnetoresistance is a general property of a material whereby its electrical resistance is dependent on the angle between the direction of an electrical current flow within the material and the direction of an externally applied magnetic field. The resulting electrical resistance is generally a maximum when the current flow and the externally applied magnetic field are parallel. To produce an electrical resistance with a linear dependence on a change of the direction of the externally applied magnetic field, conductive stripes, typically aluminum or gold, are deposited on the surface of a thin film of an appropriate magnetoresistive material, such as Permalloy, at an angle inclined to a conductive axis of the device by about 45°. Such a structure is often referred to as a “barber pole.”
The current distribution within a stripe of an xMR material is roughly uniform over its width, which is usually not the optimal arrangement in certain sensor applications. In order to obtain efficient sensor performance, an xMR stripe is generally formed with a very wide lateral dimension with respect to current flow (e.g., for angle-sensing applications) or with a very narrow lateral dimension (e.g., for rotary speed-sensing applications), which is disadvantageous in view of sensor sensitivity, size, and manufacturing process controllability.
Thus, a challenge in designing a sensor utilizing a stripe of an xMR material to sense a position, velocity, or an angle of a physical object is generating a reliable resistance change in the sensor with sufficient repeatability, magnitude, and accuracy for the application, and with low cost.